Bakcground: The antioxidant activity of quercetin has been linked to many factors including its chemical structure. Studies about the process of quercetin oxidation have related the formation of dimers through a dioxane linkage, which could result from Diels-Alder cycloaddition. To evaluate the impact of microwave irradiation on the quercetin dimers formation, Diels-Alder reactions of quercetin with 3,5-di-tert-butyl-o-quinone were investigated, and heterodimer isolation and structural determination was performed.
Methods: Reactions of quercetin with 3,5-di-tert-butyl-o-benzoquinone were completed under conventional heating conditions as well as microwave irradiation. 1D/2D NMR experiments allowed the identification and quantification of the major isomer, and the energy of the most stable conformer was obtained by DFT calculations at M06-2X/6-31G(d,p) level of theory.
Results: Under conventional thermal conditions a reaction time of 10 h allows to obtain a product mixture of isomers in approximately in 2:1 ratio (1H NMR). Although the proportion of product mixture remains similar under microwave irradiation, the complete conversion took 30 min. In addition, a 1:5 molar ratio of quercetin to benzoquinone was found to increase to 75% the formation of the thermodynamic product (DFT) 8,10-di-tert-butyl-5a-(3,4-dihydroxyphenyl)-1,3,11atrihydroxy- 5aH-benzo[5,6][1,4]dioxino[2,3-b]chromen-12(11aH)-one whose identity was established by spectroscopic analyses in conjunction with molecular modeling studies.
Conclusion: A quercetin heterodimer was synthesized by Diels-Alder reaction under classical heating and microwave irradiation conditions. The use of microwave flash heating promotes shorter reaction times and a slight improvement in stereochemistry. This synthetic methodology suggest that microwave irradiation approach has a positive impact on the mechanism of cycloaddition.